Light emitting diodes (LEDs) are solid state semiconductor devices that emit light with a narrow spectral distribution when an electric current is applied. The wavelength of light emitted by the LEDs is a direct result of the bandgap of the emissive layer comprising the quantum dot which is, in turn, related to the semiconductor composition.
High brightness blue (peak wavelength between 450 nm and 470 nm), violet (peak wavelength ˜410 nm) and ultraviolet LEDs (peak wavelength ˜380 nm) have improved in terms of both brightness, efficiency, and longevity. Green indium gallium nitride (InGaN) LEDs (peak wavelength ˜520 nm) are also available, however, the efficiency of LEDs made of this material system drops precipitously for wavelengths approaching 555 nm green.
The first and most common method to achieve white light from an LED is to combine a phosphor powder with an epoxy or silicone encapsulant and apply the mixture onto the surface of an InGaN LED chip or within a reflector cup containing a blue InGaN LED chip. The phosphor absorbs a portion of the blue light emitted by the underlying LED chip and down converts that light to a slightly longer broadband yellow wavelength. At the appropriate phosphor combination, the ratio of broadband yellow light plus the residual blue light derived from the LED chip that is not absorbed by the phosphor yields a white color. See Schotter P., “Luminescence Conversion of Blue Light Emitting Diodes,” App. Phys. A., Vol. 64, pgs. 417-418 (1997). Similarly, other specialty colors such as pink can be made by adding “red” emitting phosphors to a blue emitting LED chip. Lanthanide doped garnets, nitrides and orthosilicates are the most widely used types of phosphors for LED application. Exemplary broadband yellow phosphors used to create white light include cerium doped yttrium aluminum garnet (Ce:YAG) or cerium doped terbium aluminum garnet (Ce:TAG). A typical emission spectrum of the white light LEDs, prepared by combining the YAG phosphor with a blue light, has two distinct peaks, where the first peak corresponds to blue LED emission, ˜470 nm, and the second peak corresponds to the emission of the YAG phosphor, ˜555 nm. Generally speaking, white light made in this way is of poor color quality (low color rendering index-CRI) and can reach a limited range of white color temperatures (typically 6500-4500K). Phosphors generally have a fairly narrow absorption spectra and as such can only be used on underlying light sources having a very specific range of emission wavelengths. The Ce:YAG is optimized for 460 nm light but is poorly suited for LED chips emitting at any other wavelength.
High brightness LEDs including white and specialty color LEDs have diverse applications including traffic signals, signage and display lighting, architectural lighting, LCD display backlights used in mobile phones and PDAs, larger flat panel LCD backlights and projectors/projection TV, outdoor/landscape lighting luminaires, interior illumination in the transportation sector (airplanes, subways, ships, etc.), and automobiles. As such there is a need for bright long lasting LEDs available in a wide variety of colors.
Quantum dots (also known as semiconductor nanocrystals) can be used as down converters applied onto short wavelength LED chips and used to generate the visible and infrared light. Quantum dots are tiny crystals of II-VI, III-V, IV-VI materials that have a diameter between 1 nanometer (nm) and 20 nm. In the strong confinement limit, the physical diameter of the quantum dot is smaller than the bulk excitation Bohr radius causing quantum confinement effects to predominate. In this regime, the quantum dot is a 0-dimensional system that has both quantized density and energy of electronic states where the energy differences between electronic states are a function of both the quantum dot composition and the physical size of the quantum dot itself. Larger quantum dots have more closely-spaced energy states and smaller quantum dots have the reverse. Because interaction of light and matter is determined by the density and energy of electronic states, many of the optical and electric (optoelectronic) properties of quantum dots can be tuned or altered simply by changing the quantum dot geometry (i.e. physical size).
Single quantum dots or monodisperse populations of quantum dots exhibit unique optical properties that are size tunable. Both the onset of absorption and the photoluminescent wavelength are a function of quantum dot size and composition. The quantum dots will absorb all wavelengths shorter than the absorption onset, however photoluminescence will always occur at the absorption onset. The bandwidth of the photoluminescent spectra is due to both homogeneous and inhomogeneous broadening mechanisms. Homogeneous mechanisms include temperature dependent Doppler broadening and broadening due to the Heisenberg Uncertainty Principle, while inhomogeneous broadening is due to the size distribution of the quantum dots. The narrower the size distribution of the quantum dots, the narrower the full-width half-max (FWHM) of the resultant photoluminescent spectra. In 1991, Louis Eugene Brus wrote a paper reviewing the theoretical and experimental research conducted on colloidally grown quantum dots, such as cadmium selenide (CdSe) in particular (Brus L., Quantum Crystallites and Nonlinear Optics, Applied Physics A, 53 (1991)). That research, precipitated in the early 1980's by the likes of Efros, Ekimov, and Brus himself, greatly accelerated by the end of that decade as demonstrated by the increase in the number of papers concerning colloidally grown quantum dots.
For a given quantum dot, the emission band is dependant on the size of the quantum dot. For instance, CdSe covers the whole visible range: the 2 nm diameter CdSe quantum dot emits in the blue range and 10 nm CdSe emits in the red range.
Therefore quantum dots are useful as a novel optical down converter that, when combined with a light emitting diode light source, could produce a range of colors that are unattainable with conventional phosphors. One of the challenges to date, however, is that quantum dots are susceptible to degradation when dispersed in many polymeric materials that results in degradation of brightness. Quantum dots are also susceptible to photo-oxidation which results in permanent degradation of brightness over time when exposed to oxygen and light. Furthermore, quantum dot brightness is also reduced at elevated temperatures such as those found on the surfaces of LED chips. Lastly, the process by which quantum dots are applied to LED chips should be compatible with contemporary manufacturing processes.
Until now there were several manners in which to apply quantum dots as down converters. Bawendi et al. has demonstrated that nanocrystals may be dispersed within polystyrene solution and applied to the surface of an LED. However, this method requires that the solvent in which the polystyrene and nanocrystals are dispersed be evaporated which is incompatible with conventional manufacturing processes. This may also result in a porous nanocrystal composite that does not protect the nanocrystals from oxygen and thus enables photo-oxidative degeneration of the nanocrystals. Furthermore, polystyrene is subject to degradation (yellowing) itself under the intense light of an LED chip. Bawendi et al. also demonstrated that nanocrystals in various solvents may be added to methacrylate monomers or epoxies which react to for a polymeric solid. However again, the use of solvents results in porous films and subject the nanocrystals to photo-oxidative degradation. Those methods are also incompatible with conventional LED manufacturing processes. Rohwer et al. demonstrated white light LEDs comprising a “blue” InGaN LED chip upon which CdS nanocrystals were dispersed. The CdS nanocrystals were prepared in such a way that there existed a prevalence of defects on the nanocrystal surface that result in well known broadband surface trap emission. This light emission mechanism is inefficient and results in low efficacy LEDs. See U.S. Pat. No. 6,914,265, U.S. Pat. No. 6,890,777, U.S. Pat. No. 6,803,719, U.S. Pat. No. 6,501,091 and Rohwer L., “Development of Solid State Lighting Devices Based on II-VI Semiconductor Quantum Dots,” Proc. of the SPIE, Vol. 5366 pages 66-74.
As such, there is a need in the art for a solid state lighting devices that do degrade under the intense illumination of the underlying light source, are compatible with conventional LED packaging methodologies, do not degrade the brightness of the quantum dots and/or protect the quantum dots from photo-oxidation.